In studying fatigue and road crashes, researchers first have to find out what accidents are likely related to fatigue. To do this they have to define the characteristics of fatigue-related crashes. The next step is to apply these definitions to existing databases and estimate the frequency of fatigue-related crashes. Research that informs us about the frequency of fatigue-related crashes is not yet scientific proof that fatigue directly leads to risk. The ultimate aim would be to quantify the exact relationship between fatigue state and crash risk. To do this research needs to control for other factors that may influence the relationship between fatigue and risk such as kilometres driven. Below we describe how to recognise fatigue-related crashes, the frequency of these crashes, and the evidence concerning the fatigue-risk relationship.